It is known that a single s.c. injection of testosterone proprionate (TP) or estradiol into neonatal female rats during the critical period of brain differentiation is followed postpuberally by a syndrome characterized by anovulation, polycystic ovaries, sterility, and alterations in the normal pattern of female sexual behavior. The nature of the alterations induced in the neonatal brain following steroid administration is unknown, however, these alterations appear to be localized in certain hypothalamic and limbic structures. Several recent studies indicate that the administration of TP to neonatal female rats during the critical period of brain differentiation is followed by alterations in brain nucleic acid and protein synthesis. For example, Gorski and Shryne have shown that the intrahypothalamic implantation of cycloheximide, an inhibition of protein synthesis, partially prevented androgenization following TP administration to neonatal female rats; and in an autoradiographic study, Litteria demonstrated that early TP treatment significantly reduced the incorporation of H3-lysine into proteins of specific hypothalamic nuclei, i.e., arcuate, paraventricular, periventricular, and supraoptic. The purpose of the present investigation is to determine if alterations in brain nucleic acid and protein synthesis in the female rat following neonatal TP and estradiol administraion occur as a result of changes induced at the genomic level. This will be determined by measuring the incorporation of H3-thymidine into DNA of specific brain regions utilizing the technique of quantitative radioautography.